Yuki's first interaction with Kyo is at a very young age, when he lived at Akito's whim in the Sohma family residence, always kept near Akito. He notices that he never spends time with any of the other zodiac members, who all do things together happily. The first time he sees Kyo, he thinks to himself how pretty Kyo's hair color is and is about to timidly introduce himself when Kyo explodes in anger at the sight of him. "Someone as rotten as you oughta do us all a favor and just disappear!" are his first words spoken to Yuki, which of course don't help with Yuki's crippling self-esteem. Distraught, he can find comfort and reassurance nowhere.

Their second important encounter is that of the Plot-Important Hat, which was a baseball cap given to Kyo by Shishou. Yuki stumbles upon it one day after school, and when Kyo sees him with the hat, glares and angrily stomps away and Yuki ends up keeping it. Yuki wants to return the hat, but knows that Kyo won't take it back, Akito having made the great decision to tell Yuki about Kyo's broken home life. Yuki finds out that Kyo's mother killed herself because living with Kyo was so painful. The conclusion is that if Kyo had not been born the cat and instead the rat, things would have been better, and for this reason, Kyo holds a grudge, even telling his father that he would kill Yuki and them himself.

Yuki ~ Chapter 85
I don't know his sadness. I don't know anything. If I really did disappear form this world ... would even a little of his sadness disappear? Maybe disappearing is the first truly useful thing I can do. If my dark, useless world is such a hated place, then there really is no point to my existence.

Dejected and alone, this is how Yuki and Kyo's relationship is first established in their childhood, true to the eternal legendary animosity between a cat and a rat.